1. Field of the Invention
The present invention relates to a lightning protection fastener to prevent explosion for use in a portion where there is a possibility of the presence of flammable fuel vapor, such as a fuel tank placed in an airframe of an aircraft, in particular, in a wing or a body thereof, and also relates to an aircraft assembly and a method of manufacturing aircraft assembly component.
2. Description of the Related Art
A wing partly constituting an airframe of an aircraft generally has a hollow structure. A wing-surface panel forming a wing surface is fixed to a structural member inside the wing with a fastener member (a fastener).
In this case, the fastener member fastens the wing-surface panel and the structural member together, with a pin-shaped fastener body being inserted from outside of the wing into through holes formed on both of the wing-surface panel and the structural member inside of the wing and its tip being fixed from inside of the wing with a fixing metal part.
Other than the above, in the inside of the wing and a body part, a structural member other than the wing-surface panel and a member for fixing equipment are also fastened or fixed with the fastener member (the fastener).
In this case, the fastener member fastens both members together, with a pin-shaped fastener body being inserted into both through holes formed on both of the members to be fixed together and its tip being fixed with a fixing metal part.
Note that the number of wing-surface panels or members to be fixed is not restricted to two.
Meanwhile, in an aircraft, it is required to fully assure lightning protection measures for preventing explosion. If lightning hits the aircraft and a large current flows through the wing-surface panel or a component member of a main wing or the like, part or an entirety in some cases of the current flows through these various fastening parts described above. If the current value exceeds a limit value of a passage allowable current at each fastening part, an electric discharge called an electric arc (or a thermal spark) occurs (hereinafter referred in the specification as an arc). This is a phenomenon in which a local part at a fastening interface between members mainly made of a conductive material configuring the fastening part has a occurrence of an abrupt increase in temperature due to a current passing through a fastening part, and is molten to cause an electric discharge into the air. In many cases, a molten substance called hot particles scatters from a molten portion. In general, the inner space of the wing is used also as a fuel tank. Therefore, it is required to prevent ignition by suppressing the occurrence of an arc or sealing an arc at the time of a hit by lightning, thereby preventing a discharge of the generated arc and hot particles scattered therefrom from being in contact with flammable fuel vapor, and such an explosion prevention structure is necessary (refer to “Lightning Protection of Aircraft”, Second Edition by F. A. Fisher, J. A. Plumer, and R. A. Perala, 2004, Lightning Technologies Inc., p. 218, p. 266).
Specifically, as a lightning protection (explosion prevention) measure, it is required to take a measure of suppressing or sealing the occurrence of an electric arc as described at a portion having a possibility of being filled with flammable fuel vapor among interfaces between the fastener member (the fastener) and any of various members to be fastened thereby. Here, examples of the portion having a possibility of being filled with flammable fuel vapor include the inside of a fuel tank, the inside of a surge tank (a tank where a vent scoop, a burst disk, and others are placed) generally placed on a wing end side of the fuel tank, and the inside of fuel-system equipment, in a portion inside the wing and a body part.
To achieve the above, conventionally, a structure as shown in FIGS. 10A and 10B, for example, and as described below has been proposed. The structure is such that, a fastener member 4 penetrating through a first member 2 exemplified by a wing-surface panel and a second member 3 mounted onto the inside of the wing 1 has a fastener body 4a and a fixing metal part 4b and, inside a wing 1, a cap 6 is mounted away from these fastener body 4a and fixing metal part 4b, thereby forming a gap 7 filled with air between the fastener body 4a and the fixing metal part 4b (refer to Japanese Patent Application Laid-Open No. 2-7398).
However, in the technology described in Japanese Patent Application Laid-Open No. 2-7398, the structure is not such that the cap can be positioned with respect to the fastener member 4, and the mounting position of the cap 6 depends on an operator. Therefore, there is a possibility that the center of the cap 6 and the center of the fastener member 4 may be significantly shifted. If a space between the fastener member 4 and the cap 6 is small in the gap 7, the function (insulation) of the cap 6 is degraded. At worst, if the cap 6 is mounted as being in contact with the fastener member 4, the function of the cap 6 itself may be significantly degraded.
Moreover, the cap 6 may be mounted on the second member 3 with an adhesive 9 as shown in FIG. 10A, or may have its outer perimeter covered with a sealant (an insulating material) 10 as shown in FIG. 10B. Therefore, an adhering work or a work of applying the sealant 10 is required at a mounting site, thereby taking some work. Needless to say, the space inside the wing 1 of the aircraft is narrow, and workability of performing the work as described above at a deep position is extremely low. Furthermore, since several thousands to several tens of thousands of such fastener members 4 are provided to the entire wing 1, degradation in workability directly links to an increase in cost. Still further, sufficiently applying the sealant 10 tends to invite overweight. Other than the wing, portions having a possibility that flammable fuel vapor is present have a similar problem, although the number of fastener members 4 required in those portions is smaller than that in the wing.
Still further, the work as described above is carried out manually, and the quality of work tends to vary depending on the worker, which also influences reliability.
FIG. 11 shows an example of a general arc sealing method at a fastener part. An example of this type is also disclosed in “Lightning Protection of Aircraft”, Second Edition by F. A. Fisher, J. A. Plumer, and R. A. Perala, 2004, Lightning Technologies Inc., p. 266, Fig. 7.47, Fastener sealing concepts. (b). A fastening portion of the fastener member 4 on a side where flammable fuel vapor is present (such as a fuel-tank side) is over-coated with the sealant 10, or a cap seal 5 (a cap with the sealant hardened in advance) is bonded to that portion with the sealant for placement. However, even in this example, a work of sealant coating is required, and there is a problem similar to that described above.
Still further, the sealant is exposed to a low-temperature environment, for example, at −60 degrees Celsius, during flight of the aircraft. In such an environment, there is a possibility that the sealant is hardened to degrade intimate contact capabilities. To provide a sufficient arc suppressing capability even in this state, it may be required to apply a sufficiently thick sealant, which poses a problem in weight.
Still further, due to revisions in aviation regulations to prevent explosion of fuel tanks, demands for preventing failures in these lightning protection (explosion prevention) measures have become extremely strict, and the occurrence of a failure in even one fastener among several thousands to several tens of thousands fasteners per aircraft cannot be permitted. Therefore, it is required for the lightning protection (explosion prevention) measures to prevent the occurrence of failures at the time of manufacture (manufacturing mistakes, mount mistakes, and inspection mistakes), to prevent a decrease in capabilities due to variations in manufacture, and to prevent deterioration in performance and others in an operating environment with an aircraft life of over thirty years, and ensuring reliability and guaranteeing quality as described above are strictly demanded. On the other hand, from economical and environmental demands, measures achieving low cost and light weight are demanded.
Thus, the inventors have already suggested a technology having a structure in which an engaging part is formed at a portion of a fastener member projecting inside of an airframe, an engaged part engaged with the engaging part of the fastener member is formed at a center part on an inner perimeter surface of a cap, and the engaging part of the fastener member is engaged with the engaged part (refer to Japanese Patent Application Laid-Open No. 2010-254287).